1. Field of the Invention
This invention relates to an optical system driving device for two-dimensionally driving an optical system. Such an optical system driving device is suitably used to drive, for example, the objective of an optical information recording-reproducing apparatus for effecting recording and reproduction of information on a recording medium by the use of optical means.
2. Related Background Art
In an optical information recording-reproducing apparatus, a laser light is converged by an objective to form a minute spot on an information track and the information track is scanned by the spot. To carry out recording and reproduction accurately, it is necessary to cause the minute spot to accurately follow the information track always in its sufficiently focused state. So, in the optical information recording-reproducing apparatus, the focusing state with respect to the recording medium and the tracking state relative to the information track are always detected and when these are likely to deviate from a proper range, the objective is moved, whereby control for maintaining a proper focusing state and tracking state is accomplished. Such focusing control and tracking control are accomplished by supporting the objective two-dimensionally movably, securing a coil for focusing control and a coil for tracking control directly or indirectly to the objective, further placing the coils in suitable steady magnetic fields, respectively, and controlling the amounts of current supplied to the respective coils in conformity with the detection signals of the focusing state and the tracking state.
As the objective driving device in the optical information recording-reproducing apparatus as described above, use has heretofore been made of one comprising two sets of parallel plate springs connected in series so as to be orthogonal to each other, as proposed in U.S. Pat. No. 4,449,213. In such device, movement of the objective in the focusing direction is effected by the flexure of one set of parallel plate springs and movement of the objective in the tracking direction is effected by the flexure of the other set of parallel plate springs.
In such an objective driving device, however, for example, when one set of parallel plate springs is flexed for focusing control, a stress in the same direction is also produced in the other set of parallel plate springs, and this has led to the fault that unnecessary resonance is produced to make accurate control difficult. There has also been the fault that the structure is complicated and cost of assembly is high.
To eliminate the above-noted drawbacks peculiar to the objective driving device using the parallel plate springs, an objective driving device has recently been proposed in which the objective is supported by the use of four parallel metal wires (PCT application 8404841). FIG. 1 of the accompanying drawings is a perspective view schematically showing the construction of an objective driving device of this type. In FIG. 1, reference numeral 52 designates an objective having an optic axis in Z direction, and reference numeral 54 denotes a holding member for the objective. Reference numeral 56 designates a fixing member for supporting and fixing a movable portion including the objective. Reference numeral 58 denotes support members comprising four metal wires of the same property each having one end thereof secured to the holding member 54 and the other end secured to the fixing member 56. The support members 58 comprising these four metal wires are parallel to one another and disposed so as to define the four edges of a rectangular parallelopiped. The support members comprising these metal wires have moderate flexural resiliency. Thus, in this objective driving device, the holding member 54 is reciprocally movable over a suitable range in Z direction (focusing direction) and X direction (tracking direction).
However, in the objective driving device as shown in FIG. 1, the rigidity to the torsion about Y direction is low, and this has led to the problem that for example, where the center of gravity of the movable portion and the action point at which a drive force acts on the movable portion during the focusing control drive or during the tracking control drive do not coincide with each other with respect to Z direction and Y direction, torsional vibration 60 abput the Y-axis is liable to occur.
FIGS. 2 and 3 of the accompanying drawings are schematic views for illustrating the occurrence of such torsional vibration. That is, assuming that for example, in FIG. 2, a force 62 in X direction acts on the tip ends of metal wires 58 (i.e., the ends of the metal wires which are secured to the objective holding member 54), all the metal wires 58 are readily bent in that direction. Accordingly, when as shown in FIG. 3, oppositely directed forces act on the ends of the two pairs of metal wires 58, the respective pairs of metal wires 58 are readily bent in the directions of arrows 64 and 66, respectively, and thus the objective holding member 54 which is a rigid member secured to the ends of the metal wires 58 is readily rotated in the direction of arrow 68 about Y direction. Due to the resilient energy stored as the flexure of the metal wires 58 based on such rotation, the objective holding member 54 begins to rotate in the direction opposite to the direction of arrow 68 and thereafter, this is repeated to provide torsional vibration.
A device similar to the objective driving device shown in FIG. 1 is disclosed in U.S. Pat. No. 4,557,564. In this device, four elongated support members are arranged so as to assume substantially a V-shape as viewed from the direction of the optic axis of an optical system when the optical system is supported by the support members.
Thus, in this device, the resilient deformation of the support members is utilized for only the movement of the optical system in the focusing direction and other means is utilized for the movement of the optical system in the tracking direction, and this has led to relatively complicated construction of the entire device.
Also, in the driving device of such an arrangement, if an attempt is made to utilize the resilient deformation of the support members also for the movement of the optical system in the tracking direction, it is difficult for the support members to be deformed because the positions at which the pairs of support members are secured to the fixing member are identical to each other, and this has led to a great drive force required for the movement of the optical system in the tracking direction.